Possibility of Rare-Gas Color Centers in Ionic Crystals

It is argued that the defect consisting of a rare-gas atom at a negative-ion vacancy in an ionic crystal should trap an electron to form an electron-excess color center. The basis for this speculation is that the potential for such defects, as for the $F$ center, must approach a Coulomb form at large distances. The principal optical absorptions for these hypothetical rare-gas centers have been calculated in the point-ion approximation for alkali-halide matrices. The transitions are predicted to lie in a range from 0.9 to 1.8 eV, depending on the rare-gas and the host crystal. In all cases, the absorption lies to the low-energy side of the corresponding $F$ band and has an oscillator strength of the order of unity. Although these centers possess bound electronic states, it is not clear that they are "chemically" stable against decay into an $F$ center and an interstitial rare-gas atom. However, from studies of the conversion of $U_3$ centers to $U_2$ centers, it is argued that the hypothetical rare-gas centers may be stable in the iodides or bromides of intermediate-weight alkalis. The possiblity of observing both substitutional rare-gas atoms and ions by means of $\alpha$- and $\beta$-band-like transitions it also discussed.